DQPSK is a Differential Quadrature Phase Shift Keying modulation method. In recent years, with the improvement of speed and the increase of capacity in optical transmission systems, more and more attention has been paid to the optical phase modulation method represented by the DQPSK.
FIG. 1 shows a structure diagram of a DQPSK demodulator according to the traditional art. As shown in FIG. 1, an input optical carrier can be expressed as: Ei=E exp j[ω0t+φ(t)], wherein E refers to field strength, ω0 refers to an angular frequency of the optical carrier, and φ(t)refers to a modulation phase. The principle of the modulation of the DQPSK demodulator is: encoding information to be transmitted in a differential phase, which is expressed by Δφ, of continuous optical bits, wherein Δφ can be any value in [0, π/2, π, 3π/2]. Assumed that the phase of the k−1th optical bit pulse is θ(k−1) , if the next bit is {0, 0}, θ(k)=θ(k−1)+π; if the next bit is {0, 1}, θ(k)=θ(k−1)+π/2; if the next bit is {1, 1}, θ(k)=θ(k−1); if the next bit is {1, 0}, θ(k)=θ(k−1)+3π/2.
Based on above DQPSK modulation process, the principle of the demodulation of the DQPSK is: demodulating a received optical signal through the DQPSK demodulator to obtain two differential currents, wherein the two differential currents carry modulation phase difference of adjacent optical bits, and the transmitted bit information flow can be obtained according to the modulation phase difference. In order to reliably obtain an I-path differential current signal and a Q-path differential current signal which can be used for extracting the modulation phase difference, and then recover the transmitted information accurately, the phase difference of two arms on an I-path of the demodulator must strictly meet a demodulation requirement, namely the phase difference isπ/4+2nπ, and the phase difference of two arms on a Q-path of the demodulator must strictly meet a demodulation requirement, namely the phase difference is −π/4+2nπ; or else, extra optical signal-to-noise ratio cost will be brought.
At present, in order to precisely control the phase difference between two arms on the I-path and the Q-path of the DQPSK demodulator, a control method commonly used in the traditional art is directly collecting current signals of a balance receiver, regulating bias voltages of the I-path and the Q-path of the demodulator at the same time to minimize the maximum value of the collected current signals, in this way, the DQPSK demodulator is locked at a correct bias point.
However, the above method is usually implemented through an analog circuit, so the response characteristic of a control loop is easily influenced by external environments, the reliability is poor, and the bias point can only be locked asπ/4 and −π/4, but not other expected values.